Systems for remotely inflating and deflating vehicle tires are well known in the art. It is known that a tire which has a relatively large footprint has superior traction and increased rolling resistance as compared to a tire with a relatively small footprint. Therefore, the ability of a pneumatic tire to have sufficient traction on soft terrain such as sand and satisfactory rolling resistance on hard terrain such as a highway can be achieved by varying the inflation pressure of the tire.
Typical systems which allow the operator of a vehicle to vary the pressure in pneumatic tires according to changing driving conditions are disclosed by Turek, U.S. Pat. No. 2,634,783; Holbrook, U.S. Pat. No. 2,989,999; Ruf, U.S. Pat. No. 3,276,502 and Goodell et al., U.S. Pat. No. 4,418,737. In all of these systems, an air passageway is maintained between the vehicle tire and the adjacent nonrotating portion of the vehicle through rotary seals. Because an air passageway is maintained between relative rotating members, significant technical problems must be overcome to provide a durable system which allows the operator to easily inflate or deflate the vehicle's tires.
One particularly acute problem has been addressed by some of the prior art system in a less than satisfactory manner. It is known that continuous pressurization of the rotary seals in these systems substantially reduces the life of the seals. In U.S. Pat. No. 4,498,515 to Holtzhauser et al., a pilot operated valve on the rotating tire is utilized to isolate the tire so that fluid pressure on the rotary seals can be relieved when the system is neither inflating nor deflating the tires. The Holtzhauser system disadvantageously achieves this result by utilizing separate fluid passageways for piloting the valve and for delivering air to and from the tire for inflation and deflation respectively. Thus, two passageways are drilled or otherwise provided through structural members which must support the weight of the vehicle on the rotating tires. On vehicles designed to carry heavy loads or which can be expected to encounter large stresses, this arrangement is undesirable. In addition, the deflation route for air leaving the tire is through one of the rotary seals to a valve in the system which can be vented to atmosphere. The deflation speed of the system is limited by the size of the passageways which can be provided between the rotating members. Because two separate passageways are required between the rotating members, the maximum size of each passageway is correspondingly limited by the load bearing requirements of the rotating assembly. Thus, the deflation speed of this system is slow.
The Holbrook and Turek et al. device alleviates some of the disadvantages of the Holtzhauser system but introduce other disadvantages. The Holbrook system only requires one air supply conduit to each tire because the supply pressure used for inflating the tires is also used to operate the pilot of an inflation control valve mounted on each tire. Turek et al. utilizes a similar system. In each of these systems, applying a high pressure to the single supply conduit which is larger than any pressure to which the tires might be inflated causes the tire valve to vent tire pressure directly to the atmosphere to deflate the tires. While this is a significant advantage in deflation speed over the Holtzhauser system it is to be noted that neither the Holbrook or Turek systems allow deflation to variable pressures when using this faster deflation mode. Both Holbrook and Turek provide spring-loaded check valves which can be used to limit the maximum deflation which may occur. Furthermore, the maximum speed at which either of these systems can deflate the tire is limited by the difference between the instantaneous tire pressure and the minimum force exerted by the check valve (the minimum pressure to which the tires may be deflated). Thus, the pressure differential available to force air out of the tires is less than the instantaneous pressure available in the tires. Neither of these systems provide the flexibility to deflate the tires to a plurality of pressures as would ideally be desired in such a system.
Other systems, such as the system disclosed by Ruf allow deflation of the tires to a variable source pressure but only provide a deflation force which is equal to the difference between the instantaneous tire pressure and the applied source pressure to force air out of the tires. Furthermore, in order to achieve this capability the Ruf structure forfeits the ability to relieve pressure on the rotary seal between the tire and the adjacent nonrotating portion of the vehicle when the inflation or deflation sequence has been completed.
A further disadvantage of all of the above described prior art systems is their inability to quickly inflate the tires of the vehicle during the inflation mode. In each of the above described systems, the pressure applied to the tires during inflation is the ultimate pressure to which the tires are to be inflated. That is, if the tires are to be inflated to 50 psig the pressure applied to the tires is 50 psig. Thus, the instantaneous value of the pressure in the tires asymptotically approaches the desired, final value. The system disclosed by Goodell et al. alleviates the above described disadvantage by applying high pressure brake system air to the tires as controlled by a master-slave valve arrangement. The master valve is controlled by one of a plurality of pressures preselected by the operator and also by the pressure in a static pressure tank which is continually connected to the tires. The difference between the selected pressure and the pressure in the static pressure tank causes the slave valve to either inflate the tires with the brake system air tank pressure or deflate the tires through the rotary seals, air passageways and the master valve. Thus, while the Goodell et al. system is capable of more rapidly filling the tires than the discussed prior air systems the ability to depressurize the rotary seals is forfeited because the static pressure tank must be in continuous communication with the pressurized tires. Furthermore, deflation of the tires is routed through the rotary seals, air passageways and slave valve to an exhaust port which results in an unacceptably slow deflation speed.
From the foregoing, it is apparent that the prior art systems for remotely inflating and deflating tires are incapable of quickly inflating and deflating tires to variable pressures, while also being capable of relieving pressure on the rotary seals after the inflation or deflation cycle has been completed. It should also be apparent that the structural limitations of any one system, which allows one feature to be achieved, precludes the achievement of features shown in other systems. Furthermore, none of the systems achieve the theoretical maximum deflation speed by utilizing all of the instantaneous pressure available in the tires during deflation, to force air out of the tire to an atmospheric exhaust port.
Thus, the need exists for a central tire inflation system which can rapidly inflate tires to various pressures and which can rapidly deflate tires to various pressures selected by the operator. The system should utilize the full potential of the available instantaneous pressure in the tire to exhaust the tire, and should only utilize one fluid passage between the rotating tires and the adjacent nonrotating vehicle portion. It would also be desirable for the system to depressurize the rotating seals after the inflation or deflation cycle has been completed to improve the service life of the seals.